Field of the Invention
This invention is related to a group III nitride wafers used to fabricate various devices including optoelectronic and electronic devices such as light emitting diodes, (LEDs), laser diodes (LDs), photo detectors, and transistors.
Description of the Existing Technology
(Note: This patent application refers several publications and patents as indicated with numbers within brackets, e.g., [x]. A list of these publications and patents can be found in the section entitled “References.”)
Gallium nitride (GaN) and its related group III nitride alloys are the key material for various optoelectronic and electronic devices such as LEDs, LDs, microwave power transistors and solar-blind photo detectors. However, the majority of these devices are grown epitaxially on heterogeneous substrates (or wafers), such as sapphire and silicon carbide since GaN wafers are extremely expensive compared to these heteroepitaxial substrates. The heteroepitaxial growth of group III nitride causes highly defected or even cracked films, which hinder the realization of high-end electronic devices, such as high-power microwave transistors.
To solve all fundamental problems caused by heteroepitaxy, it is indispensable to utilize group III nitride wafers sliced from group III nitride bulk crystals. For the majority of devices, GaN wafers are favorable because it is relatively easy to control the conductivity of the wafer and GaN wafer will provide the smallest lattice/thermal mismatch with most of device layers. However, due to the high melting point and high nitrogen vapor pressure at elevated temperature, it has been difficult to grow bulk GaN crystals. Currently, majority of commercially available GaN wafers are produced by a method called hydride vapor phase epitaxy (HVPE). HVPE is a vapor phase epitaxial film growth, thus difficult to produce bulk-shaped group III nitride crystals. Due to limitation of the crystal thickness, the typical density of line defects (e.g. dislocations) and grain boundaries is at the order of high 105 to low −106 cm−2.
To obtain high-quality group III nitride wafers of which density of dislocations and/or grain boundaries is less than 106 cm−2, a new method called ammonothermal growth, which grows group III nitride crystals in supercritical ammonia, has been developed [1-6]. Currently, high-quality GaN wafers having density of dislocations and/or grain boundaries less than 106 cm−2 can be obtained by ammonothermal growth. The ammonothermal growth is an analogue of hydrothermal growth of synthetic quartz, thus is capable of growing bulk group III nitride ingot. There are a few other methods to grow bulk crystals of group III nitride [7-10]. One growth method is a flux-method, which uses group III metal and alkali metal flux. This method can also produce bulk shaped crystal of group III nitride. Yet another growth method is physical vapor transport method, which is basically a sublimation growth. Using these techniques, bulk shaped group III nitride crystal can be obtained recently.
Although wafers of group III nitride crystal can be fabricated by simply slicing the ingot of bulk group III nitride crystal, the slicing process causes damaged layer on the surface. Since the group III nitride crystal is extremely hard material, it is rather difficult to remove the damaged layer. Also, checking the surface finishing is difficult.